1. Field of the Invention
This invention relates to light sources. The invention has particular although not exclusive relevance to light sources for use in a projection system.
One form of projection system includes one or more spatial light modulators, each modulator being controllable so as to modulate an incident light beam. Each spatial light modulator is controlled by signals from an input video signal, and may be used to modulate light of a different colour, the coloured spatially modulated light beams then being combined to form a beam which is projected onto a projection screen.
2. Description of Related Art
Spatial light modulators may take various forms. One example is a liquid crystal light modulator as for example described in EP 401912. Another example is a tiltable mirror device as for example disclosed in U.S. Pat. No. 4,856,863. Such tiltable mirror devices comprise an array of mirrored elements, each element being arranged to be electrostatically deflectable between an "on" position in which light is reflected from the element onto a projection screen, and an "off" position in which the light is directed towards a beam dump dependent on address signals applied to the array. Thus a spatially modulated beam is produced comprising "white" areas corresponding to light from the "on" elements and "black" areas corresponding to light from the "off" elements. In practice greyscales are produced by temporal modulation.
In order to illuminate spatial light modulators, it is necessary to use a high intensity light source in order to provide a substantially uniform light beam. It is also necessary for the overall dimension of the projection system, and thus the light source to be relatively compact.
ILC Technology Inc of California, USA manufacture a compact, high intensity light source which may be used for projection systems incorporating a number of spatial light modulators. This light source comprises a compact xenon arc lamp arranged to operate with an input power supply of one kilowatt to produce a 5 cm diameter output beam. Such a light source, however, suffers the disadvantage that much of the light beam does not lie within the visible spectrum. Furthermore there is a limit to the amount of input power which may be supplied to the device due to problems of overheating.
In our copending International Patent Application WO 93/26034, there is described an arc lamp suitable for use as a high intensity light source in a projection system incorporating a number of spatial light modulators. The contents of WO 93/26034 are incorporated herein by reference. In the arc lamp described in WO 93/26034, a parabolic reflector is arranged to reflect the light produced by the arc into a directional light beam. Secondary reflection means are arranged to redirect part of the reflected beam to compensate for regions of the beam which are obscured by one of the electrodes of the arc lamp. Various heat sinks are provided within the light source so as to dissipate heat generated by the electrodes which define the arc.
The arc lamp described in WO 93/26034 will now be briefly described with reference to FIG. 1 which is a schematic, partially sectioned side view of the arc lamp described in WO 93/26034.
Referring to FIG. 1, the arc lamp comprises a cathode 101 and an anode 103 in a xenon atmosphere enclosed in an enclosure defined by a parabolic reflector 105 and a light emitting sapphire window 107 formed in the shape of a lens. The cathode 101 is supported by thin metallic supports 109, and is connected to a DC voltage supply (not shown). The anode 103 is connected to a battery via a conductive path through a mounting including a heat sink 113.
In use of the lamp, an arc is struck between the anode 103 and cathode 101. As the arc is arranged to be at the focal point of the parabolic reflector 105, light from the arc will be reflected by the parabola to form a substantially parallel beam directed out of the enclosure through the sapphire window 107. The light source is provided with an outer conical reflector 115, which is arranged to deflect light at the periphery of the beam towards the central portion of the enclosure to be reflected by an inner conical reflector 117 whose reflective surface is parallel to that of the outer reflector. The outer conical reflector 117 directs the light out of the window 107, thus compensating the central part of the output beam which is obscured by the presence of the cathode 101.
Heat dissipation from the lamp is improved by the presence of cooling fins 119 formed on the cathode 101. Cooling fins 121 are also formed on the heat sink 113.
A magnet 123, which may be a permanent magnet or an electro-magnet, is arranged in the anode mounting 113 so as to provide an axial magnetic field in the direction between the anode 103 and cathode 101. This magnetic field acts as a focusing field, reducing the diameter of the arc and thus reducing the divergence of the output beam. Inserts 125 and 127 of a soft magnetic material may be used to concentrate the magnetic field produced by the magnet 123.
It will be seen that this prior art light source, suffers the disadvantage that the output beam has a characteristic divergence and efficiency which is related to the size of the arc and to the focal length of the parabolic reflector 105. Thus the divergence is related to the size of the lamp and in order to reduce the divergence or increase the efficiency of the output beam, the lamp must be made larger.
It is an object of the present invention to provide a light source comprising an arc lamp which may have lower divergence and higher efficiency than have previously been possible, without the necessity of increasing the size of the lamp.